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Significant Zr Isotope Variations in Single Zircon Grains Recording Magma Evolution History

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Significant Zr Isotope Variations in Single Zircon Grains Recording Magma Evolution History Significant Zr isotope variations in single zircon grains recording magma evolution history Jing-Liang Guoa,b,1, Zaicong Wanga,1, Wen Zhanga, Frédéric Moyniera,c, Dandan Cuia, Zhaochu Hua, and Mihai N. Duceab,d aState Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China; bDepartment of Geosciences, University of Arizona, Tucson, AZ 85718; cInstitut de Physique du Globe de Paris, Université de Paris, CNRS, 75238 Paris Cedex 05, France; and dFaculty of Geology and Geophysics, University of Bucharest, 010041 Bucharest, Romania Edited by Bruce Watson, Rensselaer Polytechnic Institute, Troy, NY, and approved July 20, 2020 (received for review February 27, 2020) Zircons widely occur in magmatic rocks and often display internal differentiation processes of the silicate Earth (17, 18), the Zr zonation finely recording the magmatic history. Here, we presented isotopes of zircon might also record related messages. in situ high-precision (2SD <0.15‰ for δ94Zr) and high–spatial-resolution Recent developments of bulk and in situ analytical methods (20 μm) stable Zr isotope compositions of magmatic zircons in a have permitted studies on mass-dependent Zr isotope fraction- suite of calc-alkaline plutonic rocks from the juvenile part of the ation in terrestrial samples, including zircons (15, 19–24). They Gangdese arc, southern Tibet. These zircon grains are internally show the great potential of Zr isotope in tracing magmatic zoned with Zr isotopically light cores and increasingly heavier processes. However, available data have been rather limited and rims. Our data suggest the preferential incorporation of lighter resulted in very controversial interpretation (21, 22), such as for Zr isotopes in zircon from the melt, which would drive the residual fractionation factor α (whether greater or smaller than 1). The α 94 90 94 90 melt to heavier values. The Rayleigh distillation model can well is defined as ( Zr/ Zr)zircon/( Zr/ Zr)melt, which depicts how explain the observed internal zoning in single zircon grains, and Zr isotopes partition into the crystallizing zircon and surround- the best-fit models gave average zircon–melt fractionation factors ing melt and is thus fundamental to understanding magmatic Zr for each sample ranging from 0.99955 to 0.99988. The average isotope evolution. Inglis et al. (21) studied a highly differentiated fractionation factors are positively correlated with the median magmatic suite from the Hekla volcano in Iceland, and suggested Ti-in-zircon temperatures, indicating a strong temperature depen- an α of 0.9995, given the increasing δ94Zr (the permil deviation 94 90 dence of Zr isotopic fractionation. The results demonstrate that of Zr/ Zr from the IPGP-Zr standard) along with decreasing EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES in situ Zr isotope analyses would be another powerful contribu- Zr contents in highly evolved samples (SiO2 >65 wt%). It implies tion to the geochemical toolbox related to zircon. The findings of that zircon favors light Zr isotopes from the melt, consistent with this study solve the fundamental issue on how zircon fractionates theoretical predictions based on the Zr-coordination difference Zr isotopes in calc-alkaline magmas, the major type of magmas betweenzirconandthemelt(21).Ontheotherhand,zirconcrystals that led to forming continental crust over time. The results also extracted from FC1 gabbroic anorthosite (Duluth Complex, north- show the great potential of stable Zr isotopes in tracing magmatic eastern Minnesota) show extremely large Zr isotope variations thermal and chemical evolution and thus possibly continental (∼5‰), revealed by both in situ (15) and bulk measurements (22). crustal differentiation. The majority of bulk-zircon values are heavier than the bulk rock, Zr isotopes | zircon | magma evolution | laser ablation MC-ICP-MS | Significance crustal differentiation Zircon, a common accessory mineral in crustal rocks, records ircon (ZrSiO4) is one of the most important accessory min- plentiful and critical information on the Earth’s history. The Zerals in rocks. It occurs in a large variety of lithologies and is isotopes of its major component, Zr, could be another powerful generated primarily in intermediate to felsic igneous rocks (1, 2). but unexplored tracer. We apply high-precision, high–spatial- Its trace element (e.g., Ti and rare-earth element [REE]) and resolution, in situ laser ablation Zr isotope measurements of isotopic (e.g., U–Pb, Lu–Hf, Si, and O) abundances have provided magmatic zircons in continental arc plutonic rocks. Single zir- major information on the absolute timing, petrochronologic con- con grains show impressive internal zoning with lighter Zr ditions, and tectonic backgrounds of important geological events isotopes in the core but heavier ones toward the rim, solving a along the Earth’shistory(3–8). They are especially important for fundamental but controversial issue on how zircon fractionates deciphering the growth and reworking history of the continental Zr isotope in evolving magmas. Our results also reveal a strong crust (9), the early differentiation of the Earth (4, 10), and the temperature dependence of Zr isotopic fractionation. The Zr styles of plate tectonics through time (11, 12). isotope is thus very promising in deciphering the differentia- Magmatic zircons crystallize early in evolved calc-alkaline magmas tion history of magmatic systems and possibly the continental (1). They are relatively insoluble in crustal melts and resistant to crust through time. chemical and physical breakdown. The diffusion of major and Author contributions: J.-L.G., Z.W., and Z.H. designed research; J.-L.G., Z.W., and F.M. trace elements in zircon is also sluggish (13). These characteristics performed research; J.-L.G., W.Z., and D.C. analyzed data; J.-L.G., Z.W., W.Z., F.M., and allow zircon to preserve elemental and isotopic zoning, as well as M.N.D. performed data interpretation; J.-L.G., Z.W., W.Z., F.M., and M.N.D. wrote several generations of geochemical information within a single the paper. grain (2, 13). Compared to bulk analyses, in situ analytical meth- The authors declare no competing interest. ods could extract more detailed thermal and chemical information This article is a PNAS Direct Submission. from intragrain chemical zoning (14, 15). Since zircon has a sig- This open access article is distributed under Creative Commons Attribution License 4.0 (CC BY). nificantly higher Zr concentration (∼48 wt% as a major element) 1To whom correspondence may be addressed. Email: [email protected] or zaicongwang@ than rock-forming minerals (none to a few hundred parts per cug.edu.cn. million) (16), the Zr content of magma should be primarily con- This article contains supporting information online at https://www.pnas.org/lookup/suppl/ trolled by zircon after its saturation. Given that Zr, as one of the high doi:10.1073/pnas.2002053117/-/DCSupplemental. field strength elements, has long been used to monitor magmatic First published August 18, 2020. www.pnas.org/cgi/doi/10.1073/pnas.2002053117 PNAS | September 1, 2020 | vol. 117 | no. 35 | 21125–21131 Downloaded by guest on September 26, 2021 and an α of 1.00106 was suggested accordingly (22). While these from ∼96 to ∼90 Ma (SI Appendix, Fig. S1 and Table S2), which studies reached opposite conclusions, they both suggested that are consistent with the magmatic flare-up (100 to 85 Ma) of re- zircon crystallization would fractionate the Zr isotope composition gional arc magmatism (26). of the melt. This effect might be recorded in the growth zoning of The Zr isotope compositions of magmatic zircons were ana- single zircon grains. lyzed by femtosecond LA-MC-ICP-MS using a small spot size of The internal growth zoning in magmatic zircons is common (2), 20 μm in diameter, which assured high spatial resolution and high and once formed, it could preserve information on the subtle and precision (SI Appendix,TableS4). Isotope ratios were calibrated transient magmatic differentiation due to the negligible diffusion of against zircon standard GJ-1 or Penglai, which are homogeneous rare earth and high-field strength elements (25). Thus, high–spatial- in Zr isotope compositions shown by Zhang et al. (15). GJ-1 is a resolution in situ analyses of internally zoned zircon grains may gem-quality megacryst zircon from Sri Lanka that has been widely provide key insights into magma differentiation. Here, we apply used for U–Pb and Hf isotope analysis (29). The Penglai zircon is high-precision laser ablation multicollector inductively coupled megacrysts (up to several millimeters in length) from Quaternary plasma mass spectrometry (LA-MC-ICP-MS) and report in situ Zr potassic basalts in south China used for O and Hf isotope analysis isotope compositions of magmatic zircons from mafic to inter- (30). In order to directly compare our zircon data with previous mediate plutonic rocks, which represent the juvenile part of the bulk analyses, the 94Zr/90Zr ratios were further expressed as the Gangdese arc in southern Tibet (26, 27). The results reveal large permil deviation from solution standard IPGP-Zr (Plasma-Cal internal Zr isotope variations in single grains, showing unambiguously standard solution; lot 5131203028). This standard has been used the preference of light Zr isotope by zircons and set the base for wide as a reference standard in most studies reporting Zr isotopic com- future applications. This study further indicates a strong temperature positions and was recently cross-calibrated with different geological – influence on zircon melt Zr isotope fractionation, suggesting its po- reference materials (15, 20, 21, 23). The within-run and long-term tential in depicting magmatic thermal and chemical evolution. external precision for δ94Zr in this study are better than 0.10‰ ‰ δ94 Mafic–Intermediate Plutonic Rocks and Zircons from the (2SE) and 0.15 (2SD), respectively. The Zr values of all an- − ‰ ‰ Gangdese Arc, Southern Tibet alyzed zircons range from 0.86 to 0.41 ,withanaverage of −0.04 ± 0.39‰ (2SD, n = 237).
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